An aromatics complex is a combination of process units that are used to convert naphtha, from a variety of sources, and pyrolysis gasoline into the basic petrochemical intermediates, benzene, toluene, and mixed xylenes. In aromatics applications, the naphtha is generally restricted to C6+ compounds to maximize the production of benzene, toluene, and xylenes. The majority of the mixed xylenes are processed further within the aromatics complex, in a xylenes recovery section, to produce one or more individual aromatic isomers. As used herein, “mixed xylenes” contain four different C8 aromatic isomers, including para-xylene which is used for the production of polyester fibers, resins and films.
Additional mixed xylenes may be produced from toluene, which is of low value, and heavy aromatics (C9+ aromatics) (also referred to hereinafter as “heavies”) that are present in reformate from the naphtha feedstock. Reformate is produced by selectively reforming the naphtha feedstock, in the presence of a reforming catalyst, to aromatics and high purity hydrogen. The naphtha feedstock is first hydrotreated to remove sulfur and nitrogen compounds and then sent to a reforming unit. In the reforming unit, paraffins and naphthenes in the naphtha feedstock are converted to aromatics, with as little aromatic ring opening or cracking as possible, producing “catalytically reformed naphtha”.
To produce additional mixed xylenes from the low-value toluene and heavy aromatics (C9+ aromatics), the aromatics complex may include a transalkylation process unit that is integrated between an aromatics fractionation section and the xylenes recovery section of the aromatics complex. The two major reactions in the transalkylation process unit are disproportionation and transalkylation. The conversion of toluene into benzene and mixed xylenes is called toluene disproportionation. Transalkylation is the conversion of a mixture of toluene, C9 aromatics (A9s), and C10 aromatics (A10s) into benzene and mixed xylenes. The process reactions are conducted in a hydrogen atmosphere to minimize coke formation on a transalkylation catalyst. As there is negligible aromatic ring destruction during the process, there is very little hydrogen consumption as a result of these reactions.
The catalytically reformed naphtha and pyrolysis gasoline feedstocks contain a large amount of phenyl groups substituted with ethyl, propyl, and butyl groups (collectively referred to herein as “higher alkyl groups”). Unfortunately, alkyl groups larger than methyl are cracked off of the phenyl group during transalkylation. “Dealkylation” refers to the complete or partial removal of the alkyl group(s). The scission of these higher alkyl groups leads to higher fuel gas yield, and higher benzene rather than more valuable para-xylene yield relative to the equivalent carbon number aromatic that had greater methyl group substitution. In addition, most of the hydrogen that is consumed during disproportionation and transalkylation is attributable to the cracking of non-aromatic impurities in the feedstock and such dealkylation of the ethyl, propyl, and butyl groups from the C9 and C10 aromatics.
Accordingly, it is desirable to provide processes for increasing overall aromatics and xylenes yield in an aromatics complex. It is also desirable to provide processes that increase the overall aromatics and xylenes yield in an aromatics complex that also reduce the amount of mass lost to fuel gas, and that shift the chemical equilibrium from benzene production to xylenes production while consuming less hydrogen. It is additionally desirable to provide processes for increasing overall aromatics and xylenes yield, while increasing conversion of toluene into mixed xylenes. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.